1. Field of the Invention
The present invention relates to a pulsed light source utilizing a laser diode for generating a short pulsed light of a high repetitive frequency (200 to 2 picosecond pulse width, for example), and more specifically to a low noise pulsed light source capable of generating an optical pulse with reduced light intensity noise and to a voltage detector device utilizing the same low noise pulsed light source.
2. Description of the Prior Art
An emitted light from a laser diode (LD) changes in its wavelength and intensity as an excitation current and ambient temperature vary. The intensity of the emitted light also changes owing to the competition among longitudinal modes and owing to mode hopping. For a method of reducing such variations of the light intensity, there is known a technique wherein a photodetector element such as a photodiode (PD) detects part of the emitted light from a laser diode to estimate an error signal between a detected light intensity level and a preset one which is in turn fed back to an excitation current source which is to drive the laser diode. Such a technique has already been used for an optical pick-up of a compact disk (CD) player and so on.
However, all prior practice to reduce the variations of the light intensity was applied to a laser diode for emitting continuous wave (CW) light or direct current (DC) light. No investigation was paid up to now to the noise involved in the intensity of such pulsed light when it is generated as well as no trial to stabilize the pulsed light intensity.
On the other hand, there are many application fields in need of short pulsed light because of temporal resolution being specified by the width of pulsed light. Those fields include an electro optic sampling technique as disclosed in IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, January 1986, PP 69 through 78 in which an ultrashort optical pulse is used as a sampling gate to nondestructively measure an electric signal with use of an electro-optic (E-O) effect; a fluorescence life measuring technique as disclosed in Rev. Sci. Instrum. 59 (4), April 1988, PP 663 through 665 in which an ultrashort optical pulse is used to measure laser excited fluorescence; estimation of response characteristics of photoelectric detectors and optical integrated circuits (OE IC), etc.; time correlated photon counting using a photomultiplier, and so on, for example. A dye laser which generates a picosecond to femtosecond width pulsed light is usable for such applications from the viewpoint of temporal resolution but with a difficulty of its being large-sized. Instead of this, laser diodes are hopeful as pulsed light sources, because they have some advantages of their being simple and small-sized instructure, inexpensive in manufacture.
Now, laser diodes can generate a short pulsed light with an about 200 to 20 picosecond width, and with about 670 nm to 1.5 .mu.m wavelengths in typical, the latter emission wavelengths being varied depending upon the kinds thereof. Additionally, a second harmonic of the pulsed light from a laser diode is available to assure a short wavelength pulsed light up to 340 nm. Repetitive frequencies of such optical pulses generally range from 0.1 to 200 MHz although being different in accordance with applications. Furthermore, there are technically available GHz high repetition pulsed light and inversely about several hundred Hz pulsed light.
The present inventors have however experimentally found that use of such a high repetition optical pulse to measurements causes intensity fluctuations of the light pulse to restrict the lower limit of a measurement range, as described below. For simplicity, there will be described a measurement of transmittance of a pulsed light through a sample 10 with use of a device illustrated in FIG. 8. In FIG. 8, a laser diode 12A (refer to FIG. 9) incorporated in a laser diode (LD) pulsed light source 12 emits the optical pulse which is controlled in its repetitive frequency by an oscillator 14 (repetitive frequency 100 MHz, pulse width 50 picosecond, and wavelength 830 nm, for example). The LD pulsed light source 12 is constructed as illustrated in FIG. 9, for example, to which a bias current has previously been supplied and on which a negative pulse is applied from an electric pulse generator 12B (Hewlett Packard, 33002A Comb-Generator (registered trademark) for example) using a step recovery diode for example to drive the LD 12A.
The pulsed light emitted from the laser diode (LD) 12A impinges upon the sample 10 through a chopper 16 (chopping frequency 1 kHz, for example) driven by the oscillator 15 and is partly absorbed by and partly transmitted through the same as an output light. The output light is focused by a lens 18 and detected by a photodetector 20 composed of a photodiode (PD) for example. An output signal from the photodetector 20 is amplified by a low noise amplifier 22 and lock-in detected by a lock-in amplifier 24. A chopper signal generated by the oscillator 15 is used for a reference signal in the lock-in amplifier 24. Herein, noises produced in the photodetector 20 and in the low noise amplifier 22 have sufficiently been more reduced than that involved in the transmitted light.
An output from the lock-in amplifier 24 is fed to an output meter 26 for example and displayed with respect to the transmittance of the foregoing output light.
Herein, although in the foregoing device of FIG. 8 was made of the chopper 16 and of the lock-in amplifier 24 for lock-in detection for the purpose of the reduction of measurement noises and the improvement of measurement accuracy, such construction is unnecessary in simple measurements, in other words, an output from the photodetector 20 may be amplified and read in a direct manner. Further, the low noise amplifier 22 may be omitted and the lock-in amplifier 24 may instead be employed.
In such a device, in case where the transmittance of the pulsed light through the sample 10 is nonlinear with respect to the incident pulsed light, the incident pulsed light must be measured up to a sufficiently low level of the intensity thereof. Thereupon, a difficulty is produced of noise involved in the pulsed light emitted from a pulsed oscillation LD, which limits the lower boundary of the measurement.
Referring to FIG. 10, exemplary noise characteristics of the LD pulsed light obtained experimentally by the present inventors are illustrated, with the horizontal axis taking frequencies and the vertical axis effective values (rms) of photoelectric current noise in decibel (dB). The point 0 dB on the vertical axis indicates a short noise level defined by the square root of the number of photons involved in the optical pulse (theoretical limit). FIG. 10 therefore indicates a noise level of the LD pulsed light normalized by the shot noise level. FIG. 10 further illustrates a noise level with the prior system as indicated by a solid line A and marks X. It is understood from the figure that the noise level when the LD undergoes pulsed oscillation is greater 10 times (20 dB) or more than the shot noise level, so that the former may be reduced to the latter shot noise level.
The data illustrated in FIG. 10 is given by measuring involved noise produced when the LD 12A to be measured is driven by a driving circuit 30 constructed as illustrated in FIG. 9 using a noise fraction measuring device composed of the photodetector 20, low noise amplifier 22, lock-in amplifier 24, an oscillator (OSC) 32 for frequency sweep, a noise detection circuit 34, and a display 36.